Macrolides are multi-membered lactone rings having one or more deoxy sugars as substituents. Erythromycin, azithromycin, and clarithromycin are macrolides that have bacteriostatic and/or bactericidal activity. Ascomycin, tacrolimus, and Pimecrolimus are also macrolides.
Ascomycin is an immunomodulating macrolactam that reportedly blocks T-cell activation, inhibits cytokine release, and inhibits mast cell activation. “The mechanism of action of ascomycin is very similar to that of cyclosporin and of tacrolimus, although the three compounds have different chemical structures.” C. E. Griffiths, Ascomycin: An Advance in the Management of Atopic Dermatitis. 144 Br. J. Dermatol., No. 4,679,679 (April 2001). Ascomycin is disclosed in U.S. Pat. No. 3,244,592, which describes the compound as an antifungal agent. The use of ascomycin as an immunosuppressant is disclosed in European Patent Application No. 323865.
Tacrolimus (FK 506) is a macrolide antibiotic that is also an immunosuppressive agent. More potent than cyclosporin, tacrolimus has a selective inhibitory effect on T-lymphocytes.
Rapamycin is an immunosuppressive lactam macrolide produceable, for example by Streptomyces hygroscopicus. The structure of rapamycin is given in Kesseler, H., et al.; 1993; Helv. Chim. Acta; 76:117. Rapamycin is an extremely potent immunosuppressant and has also been shown to have antitumor and antifungal activity. Its utility as a pharmaceutical, however, is restricted by its very low and variable bioavailability. Moreover, rapamycin is highly insoluble in aqueous media, e.g. water, making it difficult to formulate stable galenic compositions. Numerous derivatives of rapamycin are known. Rapamycin and its structurally similar analogues and derivatives are termed collectively herein as “rapamycins”. On oral administration to humans, solid rapamycins, e.g. rapamycin, may not be absorbed to any significant extent into the bloodstream.
Pimecrolimus is an anti-inflammatory compound derived from ascomycin, which is produced by certain strains of Streptomyces. Pimecrolimus is sold in the United States under the brand name ELIDEL®, and is approved for use against atopic dermatitis. The systematic nane of Pimecrolimus is (1R,9S,12S,13R,14S,17R,18E,21S,23S,24R,25S,27R)-12-[(1E)-2-{(1R,3R,4S)-4-chloro-3-methoxycyclohexyl}-1-methylvinyl]-17-ethyl-1,14-dihydroxy-23,25-dimethoxy-13,19,21,27-tetramethyl-11,28-dioxa-4-aza-tricyclo[22.3.1.04,9]octacos-18-ene-2,3,10,16-tetraone. Pimecrolimus is the 32-epichloro derivative of ascomycin. Its empirical formula is C43H68ClNO11, and its molecular weight is 810.47.
The crystalline form of a solid chemical compound (or the lack of a crystalline form) affects many of the compound's properties that are important with respect to formulation as a pharmaceutical. Such properties include, for example, the flowability of the milled solid. Flowability affects the ease with which the material is handled during processing into a pharmaceutical product. When particles of the powdered compound do not flow past each other easily, a formulation specialist must take that fact into account in developing a tablet or capsule formulation, which may necessitate the use of glidants such as colloidal silicon dioxide, talc, starch, or tribasic calcium phosphate.
Another important property of a pharmaceutical compound that may depend on crystallinity is its rate of dissolution in aqueous fluid. The rate of dissolution of an active ingredient in a patient's stomach fluid can have therapeutic consequences since it imposes an upper limit on the rate at which an orally-administered active ingredient can reach the patient's bloodstream. The solid state form of a compound may also affect its behavior on compaction and its storage stability.
These practical physical characteristics are influenced by the conformation and orientation of molecules in the unit cell, which defines a particular crystalline form of a substance. These conformational and orientation factors in turn result in particular intramolecular interactions such that different crystalline forms may give rise to distinct spectroscopic properties that may be detectable by such analytical techniques as powder X-ray diffraction, solid state 13C NMR spectrometry, and infrared spectrometry. A particular crystalline form may also give rise to thermal behavior different from that of the amorphous material or another crystalline form. Thermal behavior is measured in the laboratory by such techniques as capillary melting point, thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC), and can be used to distinguish some crystalline forms from others.
U.S. Pat. No. 6,423,722 discloses crystalline forms of pimecrolimus, such as form A, form B, etc.
The crystalline form of a solid chemical compound (or the lack of a crystalline form) affects many of the compound's properties that are important with respect to formulation as a pharmaceutical. Such properties include, for example, the flowability of the milled solid. Flowability affects the ease with which the material is handled during processing into a pharmaceutical product. When particles of the powdered compound do not flow past each other easily, a formulation specialist must take that fact into account in developing a tablet or capsule formulation, which may necessitate the use of glidants such as colloidal silicon dioxide, talc, starch or tribasic calcium phosphate.
Another important property of a pharmaceutical compound that may depend on crystallinity is its rate of dissolution in aqueous fluid. The rate of dissolution of an active ingredient in a patient's stomach fluid can have therapeutic consequences since it imposes an upper limit on the rate at which an orally-administered active ingredient can reach the patient's bloodstream. The solid state form of a compound may also affect its behavior on compaction and its storage stability.
These practical physical characteristics are influenced by the conformation and orientation of molecules in the unit cell, which defines a particular crystalline form of a substance. These conformational and orientation factors in turn result in particular intramolecular interactions such that different crystalline forms may give rise to distinct spectroscopic properties that may be detectable by such analytical techniques as powder X-ray diffraction, solid state 13C NMR spectrometry, and infrared spectrometry. A particular crystalline form may also give rise to thermal behavior different from that of the amorphous material or another crystalline form. Thermal behavior is measured in the laboratory by such techniques as capillary melting point, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) and can be used to distinguish some crystalline forms from others.
The discovery of new crystalline forms of a pharmaceutically useful compound provides a new opportunity to improve the performance characteristics of a pharmaceutical product. It enlarges the repertoire of materials that a formulation scientist has available for designing, for example, a pharmaceutical dosage form of a drug with a targeted release profile or other desired characteristic.
Thus, there is a need in the art for new methods for crystallization of macrolides.